Processing the trees into engineered products involves a number of steps. One of the steps is crushing young trees (stripped of branches) to obtain loose bundles of fibrous strands. The bundles of fibers are formed into mats of crushed fibers with the fibers being generally parallel. Resin is added as well as other binding agents and the mat is dried under pressure to eventually reach a target moisture content and density. After the fibers are formed into mats and before resin is added the mats must be processed to provide a uniform density of fibers across the width W (i.e., perpendicular to the direction of the fibers, see FIG. 1) of the mat.
It is important for the mat to have a uniform density of fibers across the entire mat width W so that the resulting wood product has uniform and predictable strength. Density variation can cause failure of the wood product in use, which can have disastrous effects where the wood product is load bearing.
Old growth unprocessed wood generally has been more desirable for making engineered wood products than new growth unprocessed wood or pulpwood, in part because of the lower moisture content of older trees. Pulpwood is commonly defined as wood that is about 12-60 years of age or of a certain diameter (to be distinguished from veneer or dimension lumber). Old growth trees are rapidly vanishing as forests are depleted. New “immature” tree farms are increasing in development to provide a nearly limitless source of such wood. Such farms can grow trees at a faster rate using modern technology. Immature trees can be harvested at a younger age than old grow trees, however, there is a greater variation of fiber density in immature trees than in old growth trees, resulting in a need for improved methods of producing uniform density mats.
One type of conventional apparatus which attempts to create a uniform density mat of fibers utilizes a pair of parallel vertical opposing plates between which is inserted a mat coming off, for example, a scrim line, to be compressed (also referred to as “densified”). One or both plates are connected to a reciprocating drive mechanism which drives the plates toward each other, compressing the fibrous mat therebetween. A challenge with this apparatus is that the compressive force is applied to the front and rear edges (4, 6 in FIG. 1) of the mat proximate to the plates, but the compressive force is not evenly applied across the width of the mat. The result can be that the mat has a higher density near the front and rear edges and lower density in the middle of the mat. Additionally, a nonuniform density mat may tend to decompress over time. If the density is not consistent, the moisture content of the mat after drying is not consistent, which affects the rest of the manufacturing process and the performance characteristics of the final wood product. Delamination can result if the moisture content is too high; insufficient bonding can result if the moisture content is too low. In a veneer production process, if the mat thickness varies, the veneer thickness can vary. In a sawmill operation, the result of inconsistent density can be inconsistent wood dimensions.
It would be desirable to have an apparatus which could densify a fibrous mat uniformly across the mat and improve the resulting strength characteristics. It would also be desirable to use density to control moisture content.